1. Technical Field
The present invention relates to an image processor, an image display device, and an image processing method.
2. Related Art
In recent years, as the number of gray levels or the dynamic range of image contents increases, some dark or bright portions of a display image cannot be expressed due to insufficient contrast in a display device. Accordingly, the cases are increasing where the detail in dark or bright portions cannot be completely reproduced. In order to express such detail in the dark or bright portions, image-adaptive gray level correction processing called monochrome extension is performed.
FIG. 25 is a view for explaining the gray level correction processing. FIG. 25 schematically shows the characteristic of an image expressed by each image signal under gray level correction processing, where the horizontal axis indicates the horizontal position of the image and the vertical axis indicates the luminance level.
An input image IMG1 is an image with low luminance (low gray level) at the left side and high luminance (high gray level) at the right side. In the input image IMG1, there is a small gray level change both in the region where the luminance is low and the region where the luminance is high. When performing the gray level correction processing on the input image IMG1, gray level correction is performed according to a gamma correction curve which is convex upward so that the overall luminance is increased, as shown in FIG. 25. As a result, in an output image IMG2 after the gray level correction, luminance has increased over the whole screen. Accordingly, the luminance has increased in the left low-luminance region, but the average luminance also increases in the low-luminance regions.
As described above, in the known gray level correction process, a small gray level change in a dark portion can be expressed by increasing the overall luminance so that the luminance on the low gray level side is increased, for example, when the dark portion is included in the image. On the other hand, a small gray level change in a bright portion can be expressed by decreasing the overall luminance so that the luminance on the high gray level side is decreased, for example, when the bright portion is included in the image.
The technique related to such a gray level correction process is disclosed in JP-A-2004-266755, for example. JP-A-2004-266755 discloses the technique of calculating the gamma correction curve according to the average luminance of an input image and performing luminance correction of the input image according to the gamma correction curve. In the technique disclosed in JP-A-2004-266755, the luminance gain after correction increases as the average luminance of the input image decreases. In addition, when the input image is a moving image, flickering of the moving image after correction is suppressed by calculating the gamma correction curve on the basis of the linear sum of the average luminance of the screen and the average luminance of the previous frame.
In the technique disclosed in JP-A-2004-266755, however, since the luminance correction is uniformly performed on the whole screen, there is a problem that not only the detail in dark portions but also the whole of the dark portions or other luminance regions are expressed brightly. For this reason, when dark and bright portions are mixed, the detail in the dark portions can be expressed but the detail in the bright portions cannot be expressed, for example.
Moreover, in the technique disclosed in JP-A-2004-266755, since only the luminance component is corrected, the chromaticity of each pixel changes and the tendency of the colors on the whole screen accordingly changes. In this case, the quality of the image may deteriorate only with the correction of a luminance component. Accordingly, it is desirable to be able to maintain the tendency of the colors on the whole screen when expressing the detail of the image.